Turtle's shells are fused to their spine: you can't detatch them without killing the turtle. Snail shells grow as the nail does; they are also fused to the snail. There's a part of the snail right behind it's "foot" that secrets the proteins to make the shell. Crabs shells are exoskelitons, but they're only loosely fused to the crab's "meat". They do not grow: when the crab's flesh gets too big for the shell, the shell cracks and the crab spends hours squirming out of it, then the crab's freshly exposed 'skin' dies & hardens over the course of a few days, resulting in a new shell.

In a alltogether random tangent (yet vaguely attached to the original idea)...
did you know that the force that it takes to swing a gerbil by the tail to snap its neck on a table (The standard method for pre-killing rodents before feeding them to snakes...) is too much for its tail to with stand. You will lauch a skined-tailed gerbil through the air and be left with a tube of grbil tail fur....